1. Field of the Invention
The present invention generally relates to a surface acoustic wave element, a surface acoustic wave apparatus that has the surface acoustic wave element, and a communication apparatus that has the surface acoustic wave apparatus. In particular, the invention relates to a surface acoustic wave element as well as a surface acoustic wave apparatus that is used in or as a duplexer or used in or as a composite filter that has a plurality of filters having band pass frequency ranges that are different from one another. In addition, the invention relates to a communication apparatus that has such a duplexer or composite filter comprising a surface acoustic wave apparatus of the invention.
2. Description of the Related Art
In the technical field of a communication apparatus such as a mobile phone, a surface acoustic wave apparatus that is provided with a surface acoustic wave element, known as a SAW device, is used as a duplexer or a filter (e.g., branching filter), though not limited thereto.
When a surface acoustic wave apparatus is in operation, the temperature of an inter-digital transducer (IDT) electrode as well as its peripheral temperature measured in the neighborhood thereof goes up as the power level of an input signal increases. As one reason for such a temperature increase, oscillation/vibration loss converts to thermal energy, that is, heat, at the IDT electrode, which constitutes a part of each surface acoustic wave resonator of the surface acoustic wave apparatus. As another reason therefor, Joule heat is generated due to electrode line resistance. When the temperature of the IDT electrodes goes up, there occurs a problem of deterioration in the power durability of a surface acoustic wave apparatus because, as a result of such an IDT temperature increase, stress migration occurs and/or is accelerated, which could cause the breaking of a wire, a short circuit, or the like.
In an effort to overcome such a problem, a technique for suppressing the generation of heat has been proposed so far. An example of such a heat suppression technique of the related art is briefly explained below. In the configuration of a surface acoustic wave element of the related art, one serial resonator is divided into a plurality of serial resonators without changing the overall capacitance thereof. More specifically, a surface acoustic wave resonator that constitutes a serial arm having an electrostatic capacitance of C is split into two resonators with an electrostatic capacitance of 2C. These two resonators having the electrostatic capacitance of 2C are connected in series. In addition, it is disclosed that the number of the electrode fingers of the first stream serial resonator that are in an “engaged” state is made larger than the number of the electrode fingers of the second stream serial resonator. With such a structure, an electric current that flows, per electrode finger, through the comb electrode of the first stream serial resonator is reduced, thereby suppressing a temperature increase. By this means, the heat suppression technique of the related art explained above improves the power durability of a surface acoustic wave filter.
On the other hand, techniques for dissipating heat that is generated in a surface acoustic wave apparatus have also been proposed. However, a heat dissipation technique of the related art has a disadvantage in that it requires a member that has high thermal conductivity, in that the strength of a piezoelectric substrate decreases, and in that it does not offer sufficient heat radiation effects.
Thus, a technique for enhancing the power durability of a surface acoustic wave element/apparatus is needed.